Recently, remarkable technical progresses have been made in sensors. The sensor market has been rapidly expanding to not only industrial fields and consumer fields but also new fields such as medical fields or environmental fields. Furthermore, because of development of sensor network techniques for connecting sensors to networks, environments in which sensors are connected to networks have being established.
By connecting sensors to networks, sensor information items provided by the sensors may be utilized in almost real time, and collection of a large number of sensor information items may be facilitated, thereby allowing high-level information items, such as contextual information items, to be generated from the multiple sensor information items. Thus, there is a historical background in which new services that do not exist as conventional services may be generated.
Services that have been proposed as examples of services in such environments are as follows: “a service in which collecting and aggregating, by means of sensors, information items concerning states of use of electric power in homes or in offices allows unnecessary electric power to be visualized so as to support a reduction in the amount of emission of CO2 or in power consumption”; and “an agricultural support service in which multiple sensors that collect sensor information items concerning temperature, humidity, luminance, or the like are installed in a farm so that the farm is visualized from the collected sensor information items”.
However, in consideration of a situation in which a large number of sensors of various types will be placed in every part of the world in the future, it may be difficult to expect that all of the sensors are continuously connected to networks (the Internet). Accordingly, collection of sensor information items of sensors that are not necessarily continuously connected to networks is a significant issue in the case of forming sensor networks. Some techniques are considered as techniques for addressing this issue. Among the techniques, a delay/distribution-tolerant networking (DTN) technology is considered as a useful technique.
FIGS. 1, 2A, and 2B are schematic diagrams illustrating an example of a DTN technology. The DTN technology has been proposed as a communication scheme for realizing reliable data transfer even in a communication environment to which network technologies of the related art are not easily applicable, such an environment in which a large transmission delay occurs or an environment in which disconnection of communication frequently occurs. An overview of a DTN architecture is summarized in RFC4838. In the DTN technology, a communication environment in which a physical link does not necessarily exist at all times is supposed, and data is transferred in a store-and-forward method.
FIG. 1 is a schematic diagram illustrating an example of data transfer in a store-and-forward method. In an intermittent communication environment, physical lines are intermittently disconnected between network nodes (nodes A to D), such as routers, access points, user terminals, or servers. Accordingly, in a case of data transfer in the store-and-forward method, data to be transmitted is stored in each of nodes while physical lines are being disconnected, and, when the physical lines are connected between the nodes, an operation of transmitting all stored data is performed.
FIG. 2A is a schematic diagram illustrating an example of a protocol stack used in the Internet. In Internet architecture, as illustrated in FIG. 2A, connection between a transmission source and a transmission destination is established in an application layer. Furthermore, connection between routers that establish connection between the transmission source and the transmission destination is established in a network layer. Between the transmission source and the transmission destination between which connection is established in the application layer, data is relayed by the routers that are continuously connected via physical lines included in a physical layer. In relay devices such as the routers, transferring data in the network layer allows the transmission source and the transmission destination to communicate with each other.
FIG. 2B is a schematic diagram illustrating an example of a protocol stack used in a DTN network. In the DTN architecture, the store-and-forward method is realized using a new protocol layer that is called a bundle layer. Nodes (DTN hosts/DTN routers/DTN gateways) having the bundle layer, which are illustrated in FIG. 2B, are called DTN nodes. Further, the DTN routers and the DTN gateways are called forwarders. In the DTN architecture, an environment in which physical lines are intermittently disconnected is supposed. Accordingly, a forwarder finishes communication once in the bundle layer, and temporarily stores transfer data in a storage area thereof. Then, when physical lines are connected, the forwarder transfers the transfer data, which is stored in the storage area, to a DTN node that is adjacent thereto.
The DTN technology has originally been researched as a communication scheme in interplanetary communication. However, at present, the DTN technology is also expected as a communication method usable in the case of disaster, a communication method usable in a region in which connection to the Internet is not able to be established, or a method that collects sensor information items of sensors or the like which are not connected to a typical network.
Techniques that have been proposed in the related art are as follows: a system that controls, via a network, a unit which measures an air pollution state; a system that allows a large amount of data to be transmitted between monitor devices of a sensor network; a low-power sensor node that has a high-sensitivity wireless function; and a sensor network for which setting is facilitated or whose operation is facilitated.
Japanese Laid-open Patent Publications No. 2003-281671, No. 2006-295956, No. 2008-28756, and No. 2009-294814 are examples of the related art.